Italian restorer Sergio Fusetti looks at frescoes he believes were the work of Giotto. (Click for larger image) Photograph: Pietro Crocchioni/EPA

Art restorers working on frescoes in a forgotten chapel in Assisi believe they have stumbled across proof that stunning images found under layers of grime are the work of medieval artist Giotto.

The discovery of the artist's initials on the frescoes follows two years of restoration work in the Chapel of St Nicholas in the lower basilica of Saint Francis. The work was prompted by a 1997 earthquake that damaged the basilica.

Experts have argued that the frescoes in the chapel, which has been closed to the public and neglected for years, were at best the work of Giotto's followers in the 14th century.

But restorers claim the letters GB – standing for Giotto di Bondone, his full name – prove the cleaned-up images were his.

Are these Giotto's initials? Photograph: Pietro Crocchioni/EPA

Born in 1266, Giotto is considered a forefather of Italian renaissance art. He completed the Scrovegni chapel in Padua around 1305, while his Ognissanti Madonna hangs in the Uffizi in Florence.

"This is one of the first works of [Giotto's] artistic life and is of great importance to reconstruct the chronology of his work and that of his workshop," said chief restorer Sergio Fusetti of the Assisi frescoes.

"For many years this chapel was closed and used only rarely by the monks," said Fusetti. "Now the restoration will show the beauty and richness of the frescoes [which were] found in poor condition," he added.

The frescoes were painted when Gian Gaetano Orsini, a deacon, was buried. They were commissioned by his brother, a cardinal.

Restoration work last year on a fresco by Giotto depicting the death of St Francis in another part of the Assisi basilica revealed the face of a devil leering from a cloud, which had not been spotted before.

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